There is a system of three ground based antennas mounted on the surface of the earth which receive signals from deep space. These signals are transmitted by spacecraft orbiting bodies such as the Saturn or Mars. NASA is always trying to improve the performance of the system of ground based surface antennas. Presently, signals are transmitted from the earth to the spacecraft using an x-band uplink and signals are transmitted from the spacecraft to the ground based surface antennas on earth using an x-band down link. The x-band is at microwave frequency.
The x-band down link signal is focused by the antenna into a feed horn. The feed horn further focuses the signal into a rotating waveguide ambient load switch. The rotating waveguide ambient load switch has two positions. In the first position, the feed horn focuses the signal into a choke (also called a waveguide). The signal then passes through the choke into a low noise amplifier. The low noise amplifier is cryogenically cooled and includes an oscillating crystal for tuning to the frequency of the signal transmitted by the spacecraft. In the second position, a load element is used to calibrate the low noise amplifier.
As the signal passes from space through the earth's atmosphere, background noise is added to the signal received by the system of antennas because the atmosphere has a background amount of radiation. The background noise can be approximated and removed or filtered from the received signal by tuning the receiver in the ground based equipment to account for the background noise. The amount of background noise is variable and is based primarily on atmospheric conditions such as temperature. By filtering out background noise, data rates can be increased.
A load element simulates the amount of background noise used for calibrating the low noise amplifier. The load element has a known calibrated radiation temperature; at a certain temperature the load element has a predetermined amount of radiation. By knowing the temperature of the atmosphere or sky, the temperature of the load element can be monitored to calibrate the noise generated by the atmosphere in order to filter or effectively remove the background noise generated by the atmosphere by auxiliary instrumentation. The low noise amplifier may need to be calibrated three times per day to receive signals from different spacecraft.
Thus, the ambient load waveguide switch can be moved to either of two positions. In the first position, microwave signals are allowed to pass through a waveguide port in the switch from the feed horn to the low noise amplifier when the antenna is configured to receive signals from spacecraft. In the second position, a load element is inserted into the path before the low noise amplifier in order to calibrate the amplifier.
Unwanted additional noise is added by prior art rotating waveguide ambient load switches if the choke is not accurately and repeatedly positioned. Improper calibration results as well if the load element is not accurately and repeatedly positioned. Prior art ambient load waveguide switches disadvantageously added noise because the prior art switches had a long and tortuous signal path. Prior art ambient load waveguide switches typically have through paths of approximately ten inches. Further disadvantages of prior art ambient load switches are that the choke and load element cannot be accurately and repeatedly positioned to enable the ambient load element switches to handle the high data rates now required.